Today's integrated circuits include a vast number of devices. Smaller devices are key to enhance performance and to improve reliability. As MOSFET (Metal Oxide Semiconductor Field-Effect-Transistor, a name with historic connotations meaning in general an insulated gate Field-Effect-Transistor) devices are being scaled down, however, the technology becomes more complex and changes in device structures and new fabrication methods are needed to maintain the expected performance enhancement from one generation of devices to the next. In this regard the semiconductor that has progressed the farthest is the primary semiconducting material of microelectronics: silicon (Si).
There is great difficulty in maintaining performance improvements in devices of deeply submicron generations. Several avenues are being explored for keeping device performance improvements on track. Among these is the use of tensilely strained Si as the basic semiconducting device material. The strained Si layer is typically formed by growing Si epitaxially over a relaxed graded SiGe (Ge stands for germanium) based layer as discussed in Materials Science and Engineering Reports R17, 105 (1996), by P. M. Mooney, and in U.S. Pat. No. 5,659,187 to LeGoues et al. titled: “Low Defect Density/arbitrary Lattice Constant Heteroepitaxial Layers” incorporated herein by reference. For instance, a heterostructure consisting of relaxed Si0.7Ge0.3 capped with a thin (20 nm) strained Si layer has electron and hole mobilities over 80% higher than bulk Si. The higher mobility leads to faster switching speed, higher “on” current, and lower power dissipation. A MOSFET fabricated in tensile strained Si exhibits higher carrier mobilities than conventional MOSFET as it was shown for instance by K. Rim, et al. in “Enhanced performance in surface channel strained Si n and p MOSFETs”, Proceedings of the Twenty Sixth International Symposium on Compound Semiconductors Berlin, Germany 22–26 Aug. 1999. Fabrication of a tensilely strained Si layer is also taught in U.S. patent application titled: “Strained Si based layer made by UHV-CVD, and Devices Therein”, by J. Chu et al, filed Feb. 11, 2002, Ser. No. 10/073,562, incorporated herein by reference.
Innovations solving a problem, such as using SiGe as substrate material, often lead to unexpected complications. Such an unexpected difficulty arises in isolating devices when the substrate contains Ge. The two main device isolation schemes currently used in VLSI CMOS fabrication, local-oxidation of silicon (LOCOS) and shallow trench isolation (STI), both involve thermal oxidation of the substrate. However, thermal oxidation of SiGe based materials at high temperatures results in a high interface-state density, and defects caused by “snowplowing” of Ge. Therefore oxidation of SiGe based materials must be avoided in any isolation scheme.
A possible solution to this problem would be to implement an STI process without a grown oxide liner. However, the oxide liner is a very important part of the isolation process. It serves to round the top comers of the trench, preventing high-field regions from forming between a polysilicon over layer and the substrate. The grown oxide liner also reduces the density of interface states at the STI edges that can cause carrier depletion in these regions. The liner also can prevent dopant diffusion into the STI trench, particularly if it is grown in the presence of nitrogen to form an oxy-nitride layer. Finally, the liner reduces stress and prevents defect injection into the substrate upon subsequent thermal processing. Therefore, without the grown liner oxide, an STI process would be difficult to implement in a manufacturing environment.
Recognizing the problem, structures and methods were invented to avoid the oxidizing of Ge. One scheme consists of: a trench etched into a SiGe-containing substrate where the sidewalls of the trench are covered by a Si liner; a grown or deposited SiO2 passivation layer; and an insulating material that fills the trench, and which is also planar with the wafer surface. The benefit of this structure is that it avoids thermal oxidation of SiGe on the walls of an etched trench by using a silicon liner that has vastly superior passivation properties compared to SiGe. This STI isolation scheme is described in U.S. Pat. Nos. 5,266,813 and 5,308,785 to Comfort et al. both titled: “Isolation technique for silicon germanium devices” and both incorporated herein by reference.
However, the use of this prior art has significant drawbacks. The isolation structure is planar with the substrate top surface, when it would be desirable to have the insulating layer protrude above the surface to prevent non-uniform oxidation of the exposed Si liner, and to offset recessing of the isolation layers that can occur during subsequent processing. The thermal oxidation of the Si liner may be slower at the edge, possibly leading to enhanced breakdown of the gate oxide. This problem would be exacerbated if the dielectric in the trench were accidentally recessed, exposing the corner of the trench liner before growth of the gate oxide. If one tried to correct for the planarity of the isolation structure and attempt to make it to protrude out of the substrate top surface, then having the Si liner surrounding high up the protruding isolation can cause severe device problems. The problem is that the Si liner on the surface of the isolation structure is in a polycrystalline state, which is notoriously unsuitable for high performance devices. In a MOSFET geometry, the polysilicon on the surface of the protruding insolation would also extend continuously from the source to the drain at the edge of the device, and could cause leakage between source and drain. In this prior art there is no suggestion how one could overcome the discussed difficulties.